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⌁ Shake to start!⌁ it will start on its own slowly if you wait...
📻 This is a proper Colpitts oscillator. It is also a Variable Frequency Oscillator. (VFO)
📻 Although, due to the easy-to-fix deficiencies of EC, currently there is no voltage variable capacitance in the two diodes which go to ground.
📻 You can change the capacitance of the two diodes manually. Click on the diode and select the capacitance parameter.
📻 The value of the sum of the two diode capacitors is inversely proportional to the frequency.
📻 Don't use expensive varicap diodes. Instead, use low cost 1N4007 power rectification diodes.
📻 Diodes have higher capacitance, with lower volts.
📻 So, with zero volts applied to the cathode of diodes in the case of 1N4007 you might expect up to 15pF.
📻 Whereas @ 9 volts applied to the cathode, the capacitance will reduce to 7 pF. Although this seems a small amount, it is sufficient to make a VFO that will tune across a whole radio band.
📻 Note, that these two diodes are facing with the cathodes upwards, such that they act as variable capacitors only within this circuit.
📻 The main feature of a Colpitts oscillator, is the impedance divider which uses two capacitors.
📻 In this design, you will note that there is a potentiometer in the feedback loop. With this, you can vary the amount of gain, which stops the transistor from clipping, and allows you to obtain a proper sine wave.
📻 There is a form of tank. This is produced by the relationship of the two impedance splitting capacitors, together with the inductor that goes to ground + the C belonging to the varicap diodes.
📻 Also note, that the base of the first transistor (the oscillator transistor) is mid-rail biased. Actually, this is not necessary, you may remove the lower resistor and this circuit will still function. This is because, current passes through the base to emitter junction. This forms a kind of potential divider.
📻 The second transistor is a buffer. It is in the form of an emitter follower topology. This is essential, in order not to load down the oscillator transistor, when connecting this circuit to the next stage, whatever that may be. (At this frequency it would probably be part of a radio transmitter or receiver)
In a big way, Enjoy ☺️
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